The size and shape of miniature electronic components complicates automatic and batch processing. Typically fabricated in monolithic form with brick like shapes and squared edges, these difficult to handle components require precision handling techniques and equipment if they are to be processed efficiently.
End coating of chip capacitors provides a case in point. The ends are coated with metal to electrically interconnect the electrode layers of the capacitor. The end coating also provides a suitable site for soldering and, thus, subsequent electrical and mechanical connection to an electronic circuit. The coating is usually applied by dipping the capacitor into, or spraying a metal-based paint or sputtering metal directly onto, the end of the component.
The typical holding fixture, shown for example in U.S. Pat. Nos. 4,669,416, 4,395,184, 4,393,808, and 4,381,321 comprises a metal plate having holes lined with an elastomeric material. The holes are round and smaller than the components they are intended to hold.
According to these patents, vacuum assisted vibration equipment loads a component into each one of the regular array of funnel shaped openings in a rectangular loading plate. A bank of pins in a press then transfers the components from the loading plate to corresponding holes in the holding fixture. The components are forced into the elastomeric lined holes and are frictionally held by the expansive forces of the elastomeric material. The elastomeric material is, in turn, being compressed by the larger dimensions of the held component.
As best shown in FIG. 1, a problem arises due to the space 10 created between the round edge 12 of the holding fixture 14 and the flat surface 16 of the component. I.e., the holding fixture cannot effectively mask or protect the body of the component from also being coated with metal in the case of metal paint spraying or metal sputtering. This, in turn, can cause shorting or degradation of the component and result in a rejected part.
Existing carrier plates are usually fabricated from a rectangular aluminum plate having a size on the order of 7" by 11" by 11/32" thick. A regular array of holes is predrilled in the plate, a pattern of 51 holes by 83 holes for a total of 4,233 holes being typical. The hole size, and consequently the array size, may differ according to the size of the components to be handled.
Once the holes have been formed, the plate is coated with a compliant material that fills the holes. An elastomeric coating such as silicon rubber resin is often employed for this purpose.
After the compliant material cures, a new set of smaller holes, on the order of 0.046" to 0.110" in diameter (again depending on component size) is drilled or molded in line with the original holes, so that a coated plate with an array of lined holes results. These resulting holes, or receptor holes, are slightly smaller than the components to be handled, so that the components can be gently forced into the receptor holes and retained in place for end coating.
Thus, existing holding fixtures involve a multi-step fabrication process that is time consuming and correspondingly expensive to accomplish. In addition, special procedures must be employed to apply the compliant material to the plate, followed by a second precise drilling or molding operation to complete the receptor holes. Consequently, it is desirable to have a new and improved holding fixture design that is more convenient and less expensive to fabricate.
Once fabricated, existing holding fixtures experience abrasive wear and temperature degradation during use. This is because of the frictional force fit needed when square or rectangular cross-sectional components are wedged into the round holes. Each loading and unloading operation causes the sharp edges of the component to cut into and wear on the elastomeric material lining the holes. With continuous production use, it is not uncommon to replace a holding fixture of the type described in the above patents in a few weeks or months.
When abrasive wear and temperature degradation reaches a point necessitating repair or replacement, existing carrier plates exhibit further drawbacks. Though less expensive to repair than replace, repair requires dissolving and scrapping and otherwise removing the compliant material from the aluminum plate. Then fresh material must be applied, cured, and redrilled. This is time consuming and expensive to accomplish, and the damaged carrier plate must be taken out of service and shipped to repair facilities having the means for performing these operations. Consequently it is desirable to have a carrier plate that can be quickly and easily repaired--one enabling replacement of the compliant material by a user with a few simple operations.
Thus, it is desirable to have a new and improved carrier plate that alleviates the above mentioned concerns, while being compatible with other existing component handling equipment.